Panel surface flaw inspection

ABSTRACT

This invention relates to electro-optical sensing of form type and other defects on surfaces such as sheet metal or plastic panels. Method and apparatus are disclosed for detection and quantification of defects such as dents, creases, low spots, flat spots, etc. which are a result of the manufacturing, material handling and assembly process. Surfaces of interest are generally those of automobile body panels, (e.g. hoods, fenders), refrigerator panels, furniture panels, and aircraft panels. Similar applications exist to dies and other formed metallic or plastic parts. Both automatic and human visual methods and apparatus are disclosed. The disclosed invention is also effective on paint defects such as orange peel encountered in automotive and other applications. Assemblies of panels, such as car bodies may also be inspected using the invention, and both fixed and moving (e.g. robotic) sensor versions are disclosed.

BACKGROUND OF THE INVENTION

Sheet metal panels formed by a stamping process, plastic or non-metallicpanels formed by injection molding and other similar parts often showdents, low spots and other geometric distortions from irregularities inthe dies or molds used to manufacture the part or from handling damage.These may manifest themselves as indents, outdents, creases, buckles,spring back, high spots, low spots, dish shapes, tears and a myriad ofother similar defects which must be detected in the inspection process.Such inspection is generally done before painting or plating the parts,that is before a significant amount of additional money is spent on thepart or its assembly. However, subsequent handling and the assembly(e.g. welding) or paint processes themselves also can impart damage,requiring reinspection.

Such panels are often inspected visually in an environment of florescentlinear lighting which assists the inspector to determine the quality ofthe panel by observing distortions in the reflected image of the lights.However, the florescent lights are of relatively low contrast and in anycase, such inspection is slow and subjective. It is therefore verydesirable to have a method for rapidly, automatically and objectivelyevaluating such defects (both for audit and 100% inspection purposes).Such rapid, quantitative analysis of defects is important for control ofprocesses to achieve uniform flow of quality product in just-in-timeproduction systems and to achieve uniform acceptance standards betweenvendors and customers.

There are numerous optical methods to measure the contour of the partthat could in theory discriminate such flaws, triangulation or lightsectioning for example. Another technique along with imaging a grill orgrid of lines through the panel was described in the article byLippincott and Stark, Aug. 15, 1982, Applied Optics. A similarelectro-optical sensor actually constructed for inspection of body panelflaws was described in U.S. Pat. No. 4,394,683 by two of the inventorsand their colleagues, which patent is incorporated herein by reference.This uses deviation of grid lines imaged through the panel and a varientis described in FIG. 13 of this patent. This works reasonably well butsignal to noise levels are often low, especially on poorly highlightedpanels. In addition, relatively low angles of incidence to panels arenecessitated which makes operation difficult in many cases.

SUMMARY OF THE INVENTION

Described herein therefore are embodiments of the invention whichobviate much of the signal to noise, light power and other difficultiesobtained with grid or line image deviation systems and furthermorefacilitate sensor to part positioning with no critical focal depths orthe like. This then facilitates in-line or robotically controlledmachines which do not require the surface inspected to be in a closelycontrolled position.

While primarily aimed at sheet metal and plastic panels (e.g. hoods,fenders, doors, etc.), it is also useful on assemblies of such panels asin car bodies. The invention checks the panel for manufacturing andhandling flaws and those types of defects which are inherent to plasticmolding processes such as waves and sinks which occur in the manufactureof plastic panels.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is illustrated in the following embodiments:

FIG. 1 illustrates a line or grid image deviation embodiment of theinvention.

FIGS. 1a, 1b, 1c and 1d illustrate various line images.

FIG. 2 illustrates a swept point scan embodiment of the invention.

FIG. 3 illustrates a retroreflection, scanned beam, embodiment of theinvention.

FIG. 3' depicts two characteristic signals received on the screendepicted in FIG. 3.

FIG. 3a schematically depicts a split detector for detecting the beam inFIG. 3.

FIG. 3b depicts another embodiment of the invention using a scanned beamand retroreflector.

FIG. 3c schematically depicts a split detector for detecting the beam inFIG. 3b.

FIGS. 3d and 3e schematically depict a beam scan across a surfacewithout and with a defect, and the spot position received on a bi-celldetector and the output produced thereby.

FIG. 3f schematically depicts a collimated beam modification for theembodiment depicted in FIG. 3.

FIGS. 4a to 4d illustrate effects of various surface conditions.

FIGS. 5a to 5e illustrate flaw signals of different types and theirprocessing.

FIG. 6a illustrates a retroreflective embodiment using a point sourceand visual detection.

FIG. 6b illustrates a glasses embodiment of the point source depicted inFIG. 6a.

FIG. 7 illustrates an embodiment of the invention using one or morepoint sources with TV scan detection and a retroreflective grid option.

FIG. 8 illustrates a reflection in the FIGS. 6 and 7 embodiments.

FIGS. 9-12 illustrate in-line or off-line plant applications of theinvention.

FIG. 13 illustrates a computer defect readout according to theinvention.

FIGS. 14a and 14b illustrate circuit processing according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is broadly described as follows:

1. A method for inspecting surfaces comprising the steps of;

illuminating said surface with light, with a retroreflective materialreturning light reflected from said surface such that it is re-reflectedfrom said surface,

detecting said re-reflected light, and

determining from said detected re-reflected light any defects in saidsurface.

2. A method of detection of defects in a surface comprising the stepsof;

scanning a spot or other zone of light across a screen,

imaging said zone of light on said screen, reflected via said surface,onto an image position sensing detector,

determining, from changes in the position of said zone image as saidbeam is swept across said screen, any defects in said surface.

3. A method of detection of defects on a surface comprising the stepsof;

providing at least one illuminated line,

imaging said illuminated line by reflection from said surface onto aphotodetector,

analyzing the signals from said photodetector to determine deviations inthe position of said image of said illuminated line across a portion ofsaid surface, and

determining from said deviations, if any, any defects in said surface.

In addition, disclosed is apparatus to make visible form defects insurfaces comprising at least one light source means to illuminate saidsurface and retroreflective material means to redirect light reflectedfrom said surface such that the rereflected light from said surface canbe visually observed.

Apparatus is also disclosed to reduce the embodiments of the inventionin a practical manner.

An embodiment of the line image scan type is shown in FIG. 1. A grid orline section, the latter being shown, is placed horizontal to thesurface of the panel and deviations of portions of the line are read asthe sensor is moved relative to the panel defects. This system canemploy a scanning mirror or a robot to move the sensor back and forth,or the panel can move under the sensor as well.

In this case a linear lamp 1 enclosed in housing 2 having a slit opening3 is used as an illumination line or slit source for a surface of apanel 5 to be examined (e.g. a sheet metal car hood or door). The slitsource is viewed by solid state TV camera 6 containing a lens 8 and aphoto detector unit, e.g. matrix diode array 9, connected to aprocessing unit 12. A suitable array is a GE TN2500.

On a good panel, the deviation of points on any one line image andindeed between successive lines as the panel is scanned relative to thesensor is small in any local zone except in areas where contour lines orother known features of the panel exist. The scan of a good panel isshown in FIG. 1a. However, when a low spot, ding, dimple or what haveyou appears, the lines distort as is shown in FIG. 1b. This distortioncan be characterized by determining slopes or positioned changes of thedeviated line image. Defect parameters are put on as a function of theslope of the panel distortion, and/or its width, lengths, etc. Suchdefect indications can be obtained from the width "w" of the distortedlines or the change of frequency of the lines, or the like.

In this particular mode, it is very convenient to move the panelunderneath the sensor (comprising the line source and detector) orconversely to move the sensor over the panel. For example, if the sensoris attached to a robot, it can be programmably scanned over panels. Inthe robot program, one can ignore certain sections such as where contourlines exist, at the edges of the panel, etc.

As shown in FIG. 1, there are some other aspects to this embodiment.First of all, the line need not be horizontal, i.e. parallel to thepanel as shown but could be projected at an angle as by lamp 20 (shownin phantom). The 45° projection shown in phantom makes an excellentchoice in many cases. Indeed in some cases, it is desirable to rotatethe line over a sequence of angles (e.g. 0°-60°), for example to the newpositon of lamp 20 in order to obtain the best result. For example, therotated line image version as shown in FIG. 1 has a sharp discontinuitythat is made much much more visible on certain types of flaws when theline (or grid, see below) is angled to the panel (i.e. not at a zeroangle parallel to the surface). The good panel scan using a rotated lineis shown in FIG. 1c while a flaw detected scan is shown in FIG. 1d.

The line can also be perpendicular to the surface (90°). In this case,sensitivity is least but contrast is best. To cover an area, a grid oflines is required.

Because of the different angles of view at which flaws appear best, andin fact one can be looking across a panel or lengthwise on a panel andobtain different impressions of a defect depending on just what its formis, it is often desirable to program the robot to come at the panel fromdifferent approach angles. For example, the robot could look first alongthe panel length, then across its width, at 45° or anything else thattends to make certain defects visible. It is characteristic of manystamping defects that certain types always occur in certain places on aparticular panel due to die error, etc. Thus, one can program only tolook in these areas or at different view angles depending on what flawit is.

Because of the relationship between the distances L1 and L2 of the lightsource and of the camera to the panel, respectively, it is desirable insome cases to have this distance programmable. In this case, separaterobot arms such as robot arm 31 holding the light source and robot arm30 holding a camera can be individually programmed to vary the angles ofincidence, θ1 and θ2 as well as the distances L1 and L2 from the panel.Generally speaking, the larger L1 and L2 are , the more resolution isobtained but the contrast of the defect drops (especially true for L1).Therefore, very large distances are effective only when a smoothhighlighted surface is available, as on good painted panels, wellhighlighted (oiled) metal panels, etc. Small angles θ also help create asmooth surface view, but cause a reduction in sensitivity. At extremeangles (e.g. θ under 5°) contrast is sufficient even on raw metal panelsif L1 is short, but operation is difficult.

It should be noted that a single line is quite usefel but requires amechanical scan of the sensor relative to the panel to map out thecomplete panel. This could be done by moving the panel, the sensorunits, or both.

Alternatively or additionally, however, a grille of parallel lines (e.g.grille 21) or a grid of crossed lines can be employed in place of asingle line. In this case, one can obtain the reading simultaneouslyusing the 2 axis scanning capabilities of the TV camera. However, thelighting angles and the like are not quite as good from the multiplegrid locations as they could be from a single angle of incidence fromthe surface and indeed the total optical system can be enchanced for thesingle line more than it can for the grid or grille.

Basic problems, if any, with this FIG. 1 embodiment are the relativelack of contrast on poorly oiled panels and the requirement to utilizerelatively low angles of incidence in order to get good useablecontrast, i.e. θ in the range of 30 degrees or less. This makes a moredifficult scanning requirement given the different types of slopes inpanels. It is also generally desired to have light incident more normal(e.g. θ>50°) to the panel to facilitate the programming of the system aswell as to keep the sensor package small such that one might be able toutilize the light source and detector unit in the same package.

It is clear that for use of the single line above, one needs to store inmemory the individual line description and compare it sequentially withother lines. A similar system has been disclosed in a recent copendingpatent application by the inventors relative to contrast based (asopposed to geometric based as herein) surface flaw detection, includingsystems for doing so in real time which is necessary for high speedparts inspection (U.S. Ser. No. 525,801). U.S. Pat. No. 4,305,661 isalso a reference for flaw detection of this type as well as along asingle scan line.

In the present system, as in the other system disclosed above in U.S.Pat. No. 4,394,683, surface preparation on raw sheet metal panels isgenerally required using coatings such as highlighted oil applied ontothe surface (or other surface conditioning) such that the surfaceappears suitably reflective. In-line this is best done in such a waythat the ripples in the oil, if any, remain parallel in the direction ofscan. A system of this type for in-line has been shown in U.S. Pat. No.4,394,683 in FIG. 13.

Note that a programmable rotation motor 24 can be utilized to rotate theline grid, or grille light source into the positon of lamp 20 in aprogrammable way such that movement is linked to the inspections by thecamera which then looks for maximum defect indication. For example, on acertain defect viewed with line angles of 30°, a maximum line distortionmight be indicated. This angle of maximum effect can also be used todescribe the defect too. Different types of defects have differentangles of maximum effect in any one view.

There are several other inspection rationales relative to this systemand those described in subsequent embodiments. For example, on mostpanels one knows where the defects can exist. Therefore, one can goimmediately to those areas and look with the most effort, perhapsapproaching the panel from different directions at different anglesrelative to the panel length axis at different standoffs L1 and/or L2and with different grid rotational positions. Any or all of these andother parameters can be varied to suit the task at hand.

It is also contemplated that one can have different types of lines andgrids interchangeably on a turret which can be interchanged with asingle light source or with multiple light sources. This allows grids orgrilles at different angles and spacings to be repetatively viewed andthe best description of panel defects obtained.

Another embodiment of the invention replaces, in essence, the linesource of FIG. 1 with a single point laser scan. This occurs in twomodes. In the first mode, shown in FIG. 2, a point 100 produced by alaser 102 and a rotating mirror 103 is sequentially swept across aground glass screen 110. This creates in time sequence the "line" ofFIG. 1. Each point on that screen is then imaged sequentially onto a TVcamera 130 viewing the screen through reflection off of a panel 131. Inplace of TV camera 130, one can use a synchronizing mirror scan (notshown) maintaining the point 100 image on a linear diode array (or ananalog positon sensing detector such as a UDT Pin 2D or SC10--the analogsensors provide often more speed or range). A CRT has also been used togenerate a flying spot which worked well but intensity is weak unlesshighly sensitive detectors are used.

While mainly of use on geometric distortions of the panels, it is clearthat the camera of such a system can be used to see scratches and othersharper deformities of the panel as well using the same light source oradditional supplementary light sources to illuminate the panel surface.

Where a grid or grille is desired such as shown by grille 21 in FIG. 1,it is also possible to have a scanning unit mounted on a robot such thatthe robot only has to place the sensor unit in various fixed positionsrelative to the panel. This makes it easier to program the robot sinceit does not have to make sweeps in a uniform manner.

However, even a sweep of a line across the panel over let's say a fourinch zone can be provided by the end of an arm tooling having a separatesweep scan on it. One can also use a mirror type system to sweep thisback and forth on the surface of the part. These possibilities aredescribed relative to the analogous cases in the embodiments describedbelow.

A two axis analog point (spot) image position detector such as a UDTSC10 can also be used in place of a raster scan TV sensor 130 to obtaina much higher frequency response (e.g. 2 KHz) than the relativelylimited 30 scans/sec of a conventional TV. High speed TV cameras at 400scans/sec can also be used as can random axis TV scans such as advancedforms of the GE CID (solid state) cameras which allow only the zonewhere the point exists to be scanned.

A version using a synchronized scan with a linear diode array is capableof 1000 array scans and therefore 1000 data points/sec. Using 200 datapoints across a 10" panel strip, this represents 0.050" width per pointwhich is capable of resolving most geometric defects of interest. At7000 scans/sec, this is 5 sweeps/sec. across the 10" panel strip.Actually, 0.1" spot image zones are often sufficient, giving 10sweeps/sec. This allows a forward scan speed of at least one inch/sec.,but ofter higher scan speeds are possible as 100% coverage is notrequired to detect geometric form flaws which are usually much larger intheir effect than 0.1". Thus, a scan rate of 5 inches/sec. is typicallypossible.

In practicing the above embodiment, however, a more advantageous versionwas found which was totally unexpected. A sheet of retroreflectivematerial was used as the light "source". A laser (or other) beam wasscanned from the same side as the image sensor such that the scannedbeam was returned by retroreflection to the image sensor. While bearingsimilarity to FIG. 2, this functions much better and needs someexplanation.

Considering now the embodiment depicted in FIG. 3, a substantiallycollimated "beam" of light 200 from a laser 201 is directed by means ofa scanning mirror 202 to a retroreflective sheet 203 by first bouncingit off the test panel 205 in question. Two typical scans which arereflected from the panel to the screen are shown in FIG. 3'. Theoppositely angled slopes at the dent are caused by the slope on one sideof the dent which is opposite to the slope on the other side of thedent. Because the retroreflecting material is not perfect, not all ofthe light returns along the same path. Instead some of the light returnsover a range of solid angles that spreads a small amount relative to theinput beam. A lens 210 images the "cone" of returning light from theretroreflecting object material via a mirror (having a hole for initialbeam transmission through it) or beam splitter 213. An image positiondetector 215 senses variations in the `spot` shaped image position whichis indicative of a local perturbation 220 in the panel 205. Note thatthe laser makes an excellent source allowing high speed scans, but theinvention will function with non-laser sources.

The image positon detector can be a split or bicell detector (e.g. UDTCorporation Pin 2D), a single axis continuous detector such as a UDTLC5, a two axis continuous detector such as a UDT SC10, a discretedetector with a mask, a linear diode array, a two dimensional diodearray, or a TV camera, for example.

In order to consider a larger area of the panel, the beam can be rasterscanned with another scanning mirror to scan perpendicular to the sweepdirection.

"Noise" in the form of light scattered by the surface and reflecteddirectly back from the panel (i.e. not passing to and from theretroreflector material) can be eliminated (as is highly desirable forbest results) by using a polarized laser beam and a polarizer 230 whichis "crossed" (i.e. 90°) to the incident beam polarization blocking thepolarized direct reflection from the panel. An appropriate (e.g. 1/8wave) retardation plate 233 will permit the light returning from theretroreflector and the panel to pass this polarizer (as it is rotated90° after passing through twice). Such objectionable reflection is worseon white or light colored painted panels and highlighted unpaintedpanels. (Signals can be up to 3 times, for example, the retroreflectedlevels.) It is minimal on black or dark painted panels. Other polarizerretardation arrangements are possible to accomplish the same goals.

Another means to remove the direct reflection includes sensing thedirect reflection using a second detector (not shown) slightly off axisfrom the beam splitter 213 and subtracting that signal from the spotposition sensor output.

The transit time difference from direct reflection from the panel asopposed to light that is returning from the retroreflector path can alsobe used to differentiate this noise signal but this is extremelydifficult due to the short time interval represented (a few nanoseconds).

An additional advantage of this embodiment of the invention is that thelight coming from the retroreflector returns on approximately the samepath as the incident beam. A very high power utilization results, and abad surface then will deflect this beam so that the electronics is justlooking for a local change in position due to local form errors (e.g. aflat spot on a curved surface, or a curved depression on a quasi-flatsurface). If a split detector 216 (e.g. UDT Pin 2D), having two elementsA and B as shown in FIG. 3a, is sensing the position of the beam, thenit is the difference A minus B which gives the information. Thedifference A minus B divided by the sum A plus B gives a normalizedoutput. This tolerates a wider variation of part reflectivity withoutcausing difficulty with the signal. The sum A+B is the returned lightintensity proportional to the reflectivity of the panel.

FIG. 3b illustrates another embodiment of the invention. In thisembodiment, a light beam 300 is reflected from the normal surface of thepanel 301, reaches reflective screen 302 producing a zone 310 (e.g. aspot), and is re-emitted. Lens 320, with aperture width D, can image thespot on the screen over all areas of the surface within its field ofview which subtends a zone of the surface considerably larger than theincident beam.

For example, if the beam is directed at the screen via the normalsurface portion A, one can image from the normal surface the spot on thescreen which includes a portion from the sloped area of a large dent B.

There are then two images, A' and B', formed on the detector 330 asshown in FIG. 3c. For more moderate slopes, the two images come togetherto form a blurred image which "grows" in one direction or the other asthe beam is scanned over the sloped sides of the dent. The direction ofshift is opposite depending on slope direction--up dents being reverseof down dents for any given scan direction. This is illustrated in FIGS.5a-5e, where it is also noticed that the maximum signal amplitude fromthe analog spot position detector corresponds to the maximum slope ofthe part surface. Since both positive and negative slopes occur inscanning across a dent for example, the signal can go plus to minus.

In the center of the dent, the image can grow in both directions (atsome point resulting in no centroid shift). Thus, the image of the spoton the screen can be formed through the undisturbed portion of thesurface adjacent the dent plus both oppositely sloped sides of the dent.

Especially for large beam sizes (e.g. 0.5 cm), the beam itself can breakup as it crosses the dent causing 2 beams (or even 3) to appear on thescreen. This can result in more image formation.

The above function is, however, modified by the action of theretroreflective material chosen for the screen 302. Such material isvery directional in its nature reflecting only a small angular cone(typically) of light around the axis of the beam on its surface.

Light can only be imaged from surface zones illuminated by the lightreturning from the retroreflector, i.e. returning from a small cone orother angular zone of light about the incident beam direction or axistaken to the retroreflector. For example, at L1=2 meters, such a cone atthe lens aperture is typically 30 mm in diameter where a high qualityglass bead retroreflective material is used.

Since the retroreflector is needed to allow useable light levels to beattained at the photodetector or human eye, this then means that onlyimaging of spots on the screen can be made over small angular zones. Forlarge dents, the surface slope can thus direct the re-reflected lightcompletely off the lens for L2 large and/or D small. For this reason, itis preferable to have D as large as possible for any given choice of L2.

Indeed, one can use no lens at all but just large detectors to one sideor the other of the incident beam axis to detect the shift in positionof the re-reflected cone of light from the retroreflective material.

Where a lens is used, it is desirable to make the object distance equalL1+L2 especially if one is to separate the undesired direct backreflected light from that returning via the retroreflector. However, onecan operate over a wide range of object distances and still obtain goodresults.

As noted earlier, the returning angle of light from the retroreflectorcan be somewhat larger than the outgoing beam. This is due tocharacteristics of the retroreflector itselt, i.e. it is not a perfectretroreflector but a retroreflecting screen composed of myriads ofminute elements (usually glass beads). Furthermore, the panel (die,model, etc.) surface itself is not a perfect mirror in which case it hassomewhat spread the light from its surface anyway. Thus the `spot` onthe detector is formed by viewing through a larger area of panel surfacethan the reflector. Where variations occur locally between this largerarea and the instantaneous surface deflecting the beam, spot distortionsor spot movements occur.

Lens aperture can accept light returning over larger angular zones thanthe incident beam and this allows the detector to view image zones ofthe retroreflective material through the surface from an axis. In otherwords, one is now looking at the retroreflector not directly along thebeam path but slightly displaced from it--which works as long as theangular displacement is a few degrees or less, i.e. within the returncone of the retroreflector.

For example, if one scans across a section of surface of the part withno local slope distortion, the image on the bicell detector issymetrical. This example is depicted schematically in FIG. 3d. If,however, there is such a distortion with the slope as let us say locallyfirst downward (i.e. a down ding) and the beam hits it, it is deflectedto the retroreflector and displaced from its original position. Thisexample is depicted schematically in FIG. 3e. This is picked up as adirect displacement since the detector unit is essentially viewing thesurface from behind, i.e. through the actual unsloped portion of thesurface. In other words, we have created a local reference systemwherein the "normal" surface near the defect is used as a reference(geometric in this case) against the deviated surface. This isindicative of the signals seen.

Clearly, as it goes to the other side, the reverse occurs. We areviewing the surface through the sloped area but the beam is now bouncingoff the normal surface. It should be noted that we can shift detectoraxis position above or below the defect as well causing the detector tosee in a different way the defects that are scanned across variousslopes.

In short, it is the distortion or shifting of the image of the beam spoton the detector, due to the centroid shift created by an averagingaround the instantaneous points hit by the incident beam and thecomparison of those points to other points either in advance or behind,above or below the instantaneous point, that causes the essential changein position data that creates the signal. These various parameters canbe adjusted to provide the best results for any types of panels,defects, etc.

All of the above works as long as the panel is relatively reflective,such as painted panels or panels which have been highlighted (that iscoated with a light, free flowing oil film typically kerosene based).Any other wetting type film that makes it appear mirror-like would besuitable as well. Indeed, heavier oils such as WD-40 have been usedsuccessfully. The retroreflective embodiments can operate at much higherangles of incidence than the embodiments shown in FIGS. 1 or 2 and stillgive good signal to noise, the noise being determined by the roughnessof the surface, the oil film, etc. This is of use in utilizing a roboticpositioning system as shown below.

The flaw itselt can be characterized by looking at this normalizedsignal (and/or a processed version thereof to remove both DC and/or highfrequency, e.g. highlighted or paint ripple) and evaluating the width ofthe flaw and its amplitude in a given scan and also the extent of thisflaw as measured in the scanning direction. Indents and outdents arealso normally identifiable (see FIG. 5 below).

It is noted that reflective defects, i.e. a dark spot on a light surfaceor a dull paint or highlight job, desirably show up as surface lightreflectivity variations, not as shifts in image position. Accordingly,these conditions can be differentiated from true defect conditions.

Typical values used in an extremely successful working example of theabove embodiment are:

mirror sweep rate, 60 sweep/sec. forward and backward, effectively120/sec.;

general Scanning Corp. mirror oscillator with 2" square mirror;

width of beam on panel 0.2 inches (0.5 cm);

laser, 2 mw HeNe polarized, Coherent Model CR90-21HP;

retroreflective material, 3M Scotchlight using glass beads;

imaging lens 75 mm, F1.4;

detector, UDT Corp. PIN 2D;

angle of incidence to and reflection from panel 60°;

distance to panel L1=L2=3 ft. (0.9 m).

It is noted that with this large beam size, the unit even operates onoverly thick highlight oil conditions (e.g. WD-40) that have excessivestreaking. It is further noted that best results occur for large lensapertures which can collect the maximum amount and spread of returnedlight.

The previous embodiment of FIG. 2 essentially images a point on a groundglass or other screen through the surface of the panel onto the imagedetector unit. Clearly, if the surface of the panel had a slope to it,it would throw the light off at another angle resulting in an image spotshift on the detector.

In the FIG. 3 embodiment of the retroreflector with the light source onthe same side as the detector, the light impacts a point of the paneland ostensibly comes back from the retroreflector along the same path.Therefore, one still gets nearly full light power back--a big plus and ahuge signal improvement over the FIG. 2 apparatus which loses most ofthe light generated.

However, on the face of it, one would think that the return beam wouldnot move since it would seemingly follow the same path on its return asoutgoing to the retroreflector. In fact, however, it does move, and in avery pronounced way. This is because the retroreflective screen whichessentially re-emits with a large number of small emitters, is broaderin its re-emission angle than the angle of light projection through thepanel. Therefore an image can be formed using areas of the surface notdirectly illuminated and one can get a localized comparison of theinstantaneous spot to the area around the instantaneous spot including asloped surface of the panel. One can also compare a trailing or leadingarea of the panel to the instantaneous point, or an area offset higheror lower as well simply by changing the placement of the sensor viewingaxis relative to the output beam axis. In fact, one can use multipledetectors each comparing to a different zone and compare those.

The orientation of the retroreflecting material to the incident beam ispreferably normal to it, but neither this angle or the material positionis particularly important--a big advantage for practical use on complexcontoured panels with robots, etc. (It is much better than the FIG. 1 orFIG. 2 apparatus in this regard.) However, the sensitivity of the paneldefect detection is dependent on the distance L1 of the retroreflectivematerial (e.g. Scotchlite by 3M Company) away from the panel, thedistance L2 to the sensor, as well as the incidence angle θ to thepanel. The farther away or the larger (i.e. more normal) θ, the moresensitivity to panel geometric distortions.

The panel in question or the total inspection areas can be surrounded bythe retroreflective material such that it can accommodate reflectionfrom various panel types and slopes of the panel itself. Alternatively,the retroreflective material can be carried with the sensor portion,attached to the same member or moved in concert (e.g. by a secondrobot).

A major advantage of this invention is that the sensor, including thelaser (or other light generating means) can be held less rigidly orindeed carried by a continuously moving robot as the retroreflectorkeeps the light on the optical path for most orientations. Indeed, theretroreflector itself can be tilted substantially relative to the panelsurface and still keep the light returning on its optical path. This isof crutial importance as many panels have substantial curvature causingreflections to be directed at numerous (compound) angles as one scans.This makes the invention extremely practical in its implementation.

Another advantage is that a sensor can be constructed to project andreceive light close to normal incidence (i.e. perpendicular) to thepanel surface implying that the sensor package itself can be small andlight and easily carried by a robot if necessary. In order to facilitatethis, the retroreflecting material must surround the inspection area andbe located at all angles necessary to accommodate the variousreflections off the surface. A working system operating at θ=70° hasbeen constructed.

Another advantage is that analog spot position detectors such as bicelldetectors (e.g. UDZT Pin 2D) are very fast, low in cost, and have lownoise so that inspection time can be fast. Differential measurement oftwo detector elements sensing the positon of the imaged spot of lightgive the necessary information assuming the spot does not move too muchoff one detector element. When the output is divided by the sum of thedetector outputs, the sensor is normalized and less sensitive to generalreflectivities of the panels which can change with color, oil film, etc.Such normalization can also be accomplished with continuous analogsensors such as UDT SC-10, PIN 5D, LC10 etc.

Another advantage is that this system will work on panels which arepainted or unpainted. In the latter case, the unpainted panels aresprayed, wiped or otherwise lightly coated with an oil film to smoothover the natural surface roughness of the surface itself. It ispreferred to wipe the oil film in a direction parallel to the lateralscan direction so that the scan does not cross the ripples in the oil(which are geometric in nature and can appear as "defects" or increasegreatly the background noise level). On plastic panels, the naturalsurface finish is often high enough to require no oil coating, at leastat lower incidence angles.

Using illumination angles closer to the grazing angles will make thesurface appear to look smoother which allows one to work with roughersurfaces. However, it also can produce less sensitivity to defects,depending on the defect type in question. This invention will operate onplastic panels (e.g. RIM, SMC) without oil, but require angles generallyunder 45°.

Another embodiment of the invention modifying FIG. 3 is shown in FIG.3f. In this case, a collimated or converged beam using a cylinder lens280 is shown. (A long focal length spherical lens can also be used, ascan a cylindrical or spherical mirror.) This allows the package to befolded around while still maintaining a good sized beam sweep (e.g. 10")on the surface and while limiting the size of the quarter wave materialrequired. This is occasioned by the fact that quarter wave material isdifficult to obtain in sizes larger than 12 inches. This also makes itpossible to have a smaller width retroreflector and therefore candesirably reduce the size of the unit. (With no lens, such as lens 280,the reflected beam from a convex curved surface typical of an outerautomative body panel, such as a fender, diverges, requiring a largerexpanse of retroreflective material than the beam sweep width wouldindicate. This causes excessive sensor package size.)

As shown, a large lens 280 (or for that matter curved mirror) is placedsuch that the scanning mirror 202 is approximately at its focal lengthf_(L). This collimates, or as shown, slightly converges the swept beamonto the surface of the part in one direction. The lens 280 ispreferably a cylinder lens but can be a spherical lens of a long focallength (which effectively acts like a cylinder lens over its centralportion covered by the beam and does not do much to the beam shapeitself other than slightly focus it which is okay if not too finelyfocused on the part surface).

The beam then hits the surface of the panel, and goes through thequarter wave material 233 which now can be located at the retroreflector203 while still allowing a full 12" swath, W₁ say. It is noted that ifthis is not used to obtain a 12 inch swath with a limited 12 inchretroreflector piece, one has to locate it near the surface of the panelwhich can create a difficult constructional problem.

When, as shown, the beam sweep is converged to the retroreflector, ifthe lens then is placed near the panel, the actual sweep W₁, on thepanel can be, let us say, 16 inches while still preserving a 12 inchretroreflector and quarter wave material.

The beam path can also be folded in order to make an easily manageablesensor unit which can be utilized on the end of robots or stacked sideby side without undue space requirements.

Note that when stacked side by side, a common sheet of retroreflectivematerial and quarter wave material can be used if desired, with only thescan and detection units duplicated.

There are many additional points to mention. First of all, consider thequestion of highlight oil condition and paint finish. For example,consider FIGS. 4a to 4d which illustrate the signal of a single scan ofthe FIG. 3 apparatus across a panel with FIG. 4a showing goodhighlighting, FIG. 4b showing a relatively standard paint finish, orFIG. 4c showing two cases of bad highlighting where the oil has eithernot been applied or applied much too coarsely. A fifth example shown inFIG. 4d is that where the highlight oil is in streaks which are notrunning parallel to the direction of scan as in FIG. 4a but instead runperpendicular to the direction of scan causing the maximum distortion.This is, of course, to be avoided if possible. Plastic should also bescanned parallel to its "grain", if present (e.g. as on SMC).

First, some interesting things to point out. A well highlighted panelwith the streaks of the highlight oil which had been rubbed on the panelrunning in a direction parallel to the scan actually looks better thansome painted panels. Second, it is felt that since the sensor unit isseeing geometric distortions, the ripples in the painted panel can beconsidered to be the paint finish or in extreme cases "orange peel" andtherefore the amplitude of the ripples can be used to analyze thequality of paint.

The third thing is clearly that when one gets a minimum ripplebackground surface on a highlighted panel, one knows that the correctamount of highlighting has been applied. Naturally, if a plastic panelor some other panel without requirements for highlighting is present, ofcourse such highlighting is not required. For example, a plastic panelwith no highlight is similar to FIG. 4a or FIG. 4b and sufficient foroperation. Sometimes plastic can exhibit excess background noise (likeFIG. 4d) due to a condition called "elephant hide" which is desirable todetect.

Clearly, however, when the magnitude of the ripples becomes too great, apoor (i.e. heavy, streaky) highlight paint finish or "elephant hide"condition can be signaled simply from the AC component of the detectorsignal during a sweep using known techniques. In some cases thecomponent within a certain frequency and/or level band is chosen torepresent the highlight oil (or paint finish) contribution.

A second determinant for improper conditions is when the signalamplitude is simply low, obviously indicative of poor reflectivequalities of the surface as in the case of no highlight at all on asteel panel.

When used with highlighted panels, both of these conditions can be usedto flag areas which can create invalid data due to highlight condition.Such "flags" can be fed to a computer to cause one or more things tooccur:

1. The whole panel can be rejected and a re-look made after suitablehighlighting.

2. The system can be used to help evaluate whether the highlight job iscorrect before making an analysis.

3. Particularly in 100% inspection in-line, the zone where the badhighlighting occurs can be blocked out of the computer memory and simplyignored so that the panel is not rejected for what probably is noproblem with the actual surface, only the highlight. Indeed a specialnotation can be made such that the next panel is purposely inspected inthis particular sector so that statistical data can still be built up.Naturally, if bad conditions could occur in this particular sectorrepeatedly in an in-line case (for example where an automatic highlightsystem is used such as shown in U.S. Pat. No. 4,394,683, FIG. 13, orotherwise) it can be then ascertained that something has gone wrong withthe automatic highlighter as is clogged nozzles, broken brushes and thelike and these conditions corrected.

FIGS. 5a to 5e illustrate signals of different flaws produced by theFIG. 3 apparatus. As can be seen, the type of dent in or out from thenormal surface can be found from the signal direction. For the largerdefects such as an approximately 5-10 cm wide low spot, the signal isspread out in the direction of scan. Knowledge of what magnitude, sizeand defect type(s) is present is invaluable in correcting processdefects.

FIG. 5a also illustrates processing steps according to the invention. Inthe apparatus of FIG. 3, two signal processing steps are used. In thefirst, the signal is AC coupled to remove the DC frequency component ofthe surface. Next, the threshold V_(T) is set above the maximum valueV_(N) max of the frequency components of background "noise". Thesecomponents are indicative of the paint surface or the highlight surfacesurrounding the defect and, if excessive, can indicate an invalid signalreading in the area affected if the threshold is set at normal limits.Conversely, they can also be used to measure the quality of paint,finish or highlight. For example, the value of the average noise signalV_(NA) gives an average value of the surface finish in the zone ofinterest. Orange peel ripple etc., can be detected when the signalexceeds some threshold V_(T).

The image seems to have in FIG. 5a a positive going rise followed by azero crossing and a negative portion. This is for an out ding. An inding is the reverse (for a given scan direction) as shown in FIG. 5b.

In order to determine immediately the case at hand, there are two piecesof data, the height amplitude V_(D) of the signal (with only thosesignals accepted above the background surface noise threshold V_(T)) andthe width of the defect. The latter can be obtained from the trace or bylooking at the number of successive scan lines where the defectappeared. Since this is for any one scan as we scan down a part, we canmap out in essence the defects, by storing for each given scan theamount of defects shown in terms of a code as to where on the panel theyappeared and coded to the type they are, the severity, and the widthand/or length. In this way a table can be built up in the computer whichcan be printed out.

In a typical example on a black painted hood with the FIG. 3 apparatus,V_(A) varied from 8 (small dirt in die) to 25 (severe dent) and V_(N)was 2 illustrating the excellent signal to noise indication.

A variation is to take the derivative of the signal to obtain the rateof change of the slope of the part. This is easy to obtain as a signaland also gives a distinct output proportional to the severity of thedefect.

Other processing approaches are sometimes possible for large low spots,recoils etc. Shown in FIG. 5c is the signal from a defect which is widebut shallow and which does not provide the sharp second derivativesignal.

One processing technique in this case is to correlate the characteristiccurve produced to stored low spot signatures. A scope trace (FIG. 5e)shows the correlation peak (phase delayed) indicative of a typical lowspot on the front of a hood. By tuning the frequency of the correlation,a maximum correlation signal for any low spot (or other defect) can beobtained. Since such tuning takes time, it can be desirable to identifysuch defects and come back to them, or to correlate such signals (inhardware or software) after the fact by storing them.

By using a longer illumination light source wavelength, into the infrared for example, one can eliminate the requirement for using ahighlighting oil on the surface as the longer wavelength will not be assensitive to the natural surface roughness of most materials ofinterest, e.g. steel, plastic, or aluminum. For example, at 10.6 μm (CO2laser wavelength) a steel panel looks 20 times smoother than it does at6328 A (HeNe laser).

For example, consider that a waveguide CO2 laser such as a 20 wattLaackman type could be utilized in the FIG. 3 drawing together withsuitable IR retroreflective material (e.g. glass beads to 3 μm, machinedmetal at 10.6 μm; or in the FIG. 2 drawing with suitable dispersivematerial, such as IR "ground glass") and suitable infra red optics toform the image on a Pyroelectric Vidicon having a pair of adjacent IRdetectors (arranged like 216), etc. At these wavelengths, the surface isfully reflecting and no special oil films would be required. This is abig plus in practice. IR can also be used as the light source in theembodiments depicted in FIG. 6/7 as well. Solid state or other efficientpoint IR sources can also be used.

An advantage of the invention is that the operator can view the scannedreflected information coming back from the retroreflector, eitherthrough the beam splitter or by viewing slightly off axis of theincident beam. This permits him to visually see exactly what thedetector is looking at, to confirm what the electronics is seeing. Inthis case, it is often advantageous to slow the mirror scan down.

For some flaws, it is desirable to rotate the sensor scan direction andpass over the flaw again to confirm its existance and description.

It is noted that in FIG. 3, the scan on the panel surface need not beback and forth, but can for example be circular, spiral, x shaped, etc.The circular scan offers an advantage in that it produces a smoothsignal output with no turnaround point which is useful for takingderivatives. A circular sweep, for example, can get close to certainpanel features and has no signal discontinuities which are disturbing tosensitive circuits. However, a rotating faceted mirror or oscillatingmirror scan is the preferred means of generating scans in general, whichare preferably parallel to one of the major axes of the panel.

All wavelengths visible, UV and IR of electromagnetic radiation arepossible for an illumination source. HeNe or semiconducting diode lasersare preferred but conventional sources or other lasers can be used.

A further advantage results from the fact that the light can be focussedor defocussed via optional lens 240 onto the panel to a greater orlesser degree depending on what size flaw resolution is necessary. A rawlaser beam (e.g. 0.050" wide) may be sufficiently small to detect highfrequency variations due to "orange peel" in the painting process itselfor to discern scratches, small pits and pimples, etc. A defocussed beamwill only resolve slower changes in the panel and provide more signal tonoise for low spots etc.--at a price of dimished scratch determination.This permits the system to be optimized for the case at hand. Often, adual system is desirable. Such a dual system could utilize two beamshaving different spot sizes, or use a single beam to make one completepanel analysis and then change the spot size of the single beam andrescan. In addition, sequential scans can be with different sizes byturning lasers on and off.

In an embodiment utilizing two simultaneous beams and two detectorunits, the two beams are each of a different wavelength such thatfilters in front of each detector unit can separate one from the other.Alternatively, they can be staggered in position such that one detectoronly sees one or the the other. (One beam is slightly ahead of the otherbut driven by same scanner--if sufficiently far ahead, no wavelengthdiscrimination is required. Indeed, both could be derived from the samelaser.)

A different lens can be utilized to form each beam, one for example toblow the beam up a little bit to cause it to average over ripples in thesurface due to orange peel etc. and the other one focussed down in orderto see scratches.

Therefore, not only can two spatial sensitivities be defined during asimultaneous scan, but the beam and detector channel looking at thelarger surface zone can be used as a reference level detection for thesmaller detected zone if they are both looking at the same section ofthe surface at the same time (or suitably time delayed to create thesame effect).

Note that a line array of light emitting diode light sources or fiberoptic light sources can be used instead of a sweep for illumination.Since these would likely be fixed in location, the resolution would be afunction of the spacing. However, flaw discrimination is still possible.For this version, a TV or other 2 axis scan camera is required since thesources are displaced, as is then the retroreflection. A CRT spot sweptacross its faceplate also provides such a source.

An important alternative embodiment in this invention is a manualversion depicted in FIG. 6a. This embodiment uses a substantially pointlight source 500, such as a fiber optic end connected to a halogen bulb(not shown) at the other end, near the operator's eyes 502 forilluminating the panel 505. The operator views the light returning fromthe retroreflector 510 and off of the panel. With one eye, defectsappear as dark spots on the panel. With 2 eyes, a kind of stereo occurs.For maximum results, two illumination light sources (e.g. 500 and 501)are arranged above or below each of the operator's eyes. This permitshim to have the highest signal levels at each eye (since there is noangular difference between such source and the respective eye). Theretro effect is so directional that the two sources don't interfere.

The effect produced is truly startling. To a die or stamping person itis much like when one sees a hologram for the first time. From adistance L2 of say 3 meters and L1=2 meters on a painted or wellhighlighted panel, all of the low spots and other localized geometricdistortions and imperfections in the panel appear instantlyvisible--even ones that are less than 0.01 mm deep

This effect has far reaching implications besides the use on panelsthemselves. For example, it can immediately be used to analyze paintedcars on the line in final inspection. Second, it can be used on suitablyprepared wood die models or clay models to see such distortions beforethey are scanned for CAD data. Third, it can be used to analyze dies andmolds, male or female, and instantly see where material needs to beremoved to make a smooth, good looking surface.

Substantially point light sources can be, for example, LED's,incandescent bulbs (e.g. a grain of wheat "bulb") or fiber optic endswith remoted light sources. Broad light sources such as florescent tubescan less preferably be used. These work if the tube is parallel to thesurface. The retroreflecting screens or painted retroreflecting surfacespreferably surround the inspection zone for minimum inconvenience in theinspection process and to maximize signal to noise levels.

The inspector seeks the maximum defect sensitivity position and can movehis viewing angle to achieve the best signal to noise response. Notethat the light source(s) can be located on glasses, a helmet or a headfixture so as to easily move with the operator while keeping the sourcesnear the eyes to allow for maximum retroreflective operation. Measuringreticles such as 535 (see FIG. 6b) or other aids superimposed in theoperator's vision can aid in defect size evaluation.

FIG. 6b illustrates a pair of eyeglasses provisioned according to theinvention. The frame 530 has holes in it for vision with the eye 529(only one eye shown for clarity). Light source 531 is located on the rimas is an optional second or other additonal source 532 for this eye.Optionally, a ring light source(s) surrounding the (or each) eye can beused which gives the most even illumination.

Where highlight oil is used on bare metal, it is often desirable topolarize the outgoing light from source 531 with a polarizer 540 and touse a crossed polaroid 541 in front of the eye. By virtue of quarterwave plate 550 (shown in FIG. 6a), only retroreflected light issubstantially allowed to be seen. Element 541 can also represent adefocussing or blurring device to smooth the image of minor dropletdeviations in the highlight oil as discussed below.

FIG. 7 illustrates a version of FIG. 6a operating on a female panel diein which the eye is replaced by a TV camera 600. In this case, a singlelight bulb souce 601 essentially illuminates the retroreflective screen602 via die surface 603 and the TV camera views the retroreflectivescreen through reflection from the die. As in the human eye case, thisis different than the version of FIG. 3, even though the retroreflectoris used. In this case, it's not the beam position that varies sincethere is no beam per se. Due to the same geometric distortion factors,it is the concentration or diffusion of light due to the multipleindividual "beams" from the point source that causes the image to beeither dark or bright in certain areas depending on whether the flaw isor is not present.

For example, if there is no flaw present (e.g. a high spot 616 whichshould be removed) virtually all of the light going out from the pointsource hits the retroreflector (at an angle due to the compoundingeffect spreading from the light source and of the curvature of thepanel) and comes back along the same path creating a nearly uniformlight field image of the retroreflective screen on the TV camera. If,however, there is a defect as shown at high spot 616, the light does notcome back in quite the same way and certain areas of the defect appeardarker or lighter than the surrounding area. The degree of light fieldmodification is proportional to the defect and the shape and the area ofthe defect can be immediately determined since the TV camera is capableof scanning the intensity field in two axes. Alternatively, a line scancamera can be moved relative to the surface just as in the case of thelaser scan shown in FIG. 3 creating the same effect in time sequence.

For automatic detection, it is desirable to compare light in the defect(instantaneous level) to its surroundings. A means for doing this isdescribed in a U.S. Pat. No. 4,305,661 and configurations thereof.

It is noted again that the TV camera system has less apparent ability(at least with a modest signal processor) to determine defects than ahuman which is quite good at seeing subtle light intensity gradients andthe like. Therefore, the background reflections from the surface itselfcan be a problem. In this case, crossed polarizers and a retardationplate can be used as in FIG. 3 and 6b to kill the direct back reflectionfrom the panel. As in FIG. 3, this may limit the field of view of thecamera since retardation plates larger than let us say one foot indiameter are relatively rare. Optics as in FIG. 3f can be used to expandthe field. Other techniques such as discussed above relative to the FIG.6b can also be used.

Just as in the visual case, a second TV camera unit 630 can be used toobtain a sort of binocular stereo image of the defect. In this case,each point of radiation in one image is correlated to the same point inthe other image particularly in the defect zone. This can be used toautomatically calculate the depth of the flaw condition.

It is noted that the commercially available image processor computer 640hooked to the TV camera can be used to analyze the area, shape, andintensity characteristics of the defect images in order to determinedefect parameters. The TV camera can also be used with the visualinspection and then bore sighted with the direction of view of thevisual inspection to provide a digitized analysis and quantitativeoutput of the defect being observed (as in a gunsight reticle). In thismode, the operator looks at the panel with glasses as shown in FIG. 6band the TV camera then automatically digitizes those flaws desired justby "looking" at them.

A suitable image processing computer to find display or quantify flawareas, shapes, parameter outlines, and other parameters is a MachineIntelligence Co. Model 100, a Machine Vision International Co. Genesis2000, or a GE Optimation II processor. The latter two are high speed andcapable of realtime operation. For high speed measurement on movingparts (e.g. paint cans moving on a line), strobe illumination using aflashed Xenon source for example can be used to "freeze" the image forlater analysis.

Note too that a videotape unit 660 can be optionally used to recordpanels or cars passing a line location for later analysis eithervisually or automatically. This allows a more relaxed human analysis(e.g. in an office) or a higher power large remote computer to bebrought to bear on the image defect analysis--e.g. on the 3rd shift sostatistical data would be available in the morning on the previous day'sproduction.

Another embodiment of the invention related to FIG. 1 but using theretroreflective idea presented in FIG. 3 etc. is also shown in FIG. 7.

In this optional case, however, one or more edges of a grid or grille ofparallel opaque lines or dots 620 are utilized. The grid or grille isplaced in front of the retroreflector 602, or conversely the grid orgrille is made out of retroreflective material and is used as theretroreflector. This grid then acts very much [for example, at least inthe grille or grid case] like the grille or grid described in FIG. 1except for the fact that it is illuminated retroreflectively through thepanel. The dots simply represent the grid intersection points. There isa statistical evidence that dot image centroid shifts due to defects canbe better defined than lines.

The line or grid embodiment of FIG. 7, while related to the FIG. 2embodiment, differs in that it uses the point of light being directedback from the surface. The point of light is seen as coming from a sideopposite from the sensor, but it is actually being illuminated from thesensor side. In short, while it is related to the FIG. 1 embodiment, ithowever is vastly simpler and more efficient to produce such an effect.Contrast is also much better. One needs only to have a small lightsource and a retroreflective screen with grids and indeed in this casethe screen can be one of the walls of a particular area surrounding theplace where the analysis is to take place. No particular lightingstructure or anything else is required and the power levels required arequite small. This is because the light and the camera unit are atsubstantially the same location. Light is thus not required to light thewhole room in order to be seen from a camera unit. Deviations in thepanel are also enhanced by the effects of the double reflection. Theedge points shift similar to the spot image of FIG. 3. Note that theedge deflections are easy to monitor with a TV camera. Just as in FIG.6a, if one is 10 ft. away, the whole panel can be seen superimposed overwhat are the grid lines which are geometrically distorted locally in thepresence of defects in surface form.

FIG. 8 illustrates one mechanism for defect determination in theembodiments of FIGS. 6a and 7. As shown in FIG. 8, light is incident ona defect [in this case] whose extension is illustrated as beingsubstantially in the direction of illumination rather than in thedirection transverse thereto as was shown in previous embodiments. Lightsource 700 illuminates panel 702 along axis 703 via beam splitter 704.Human eye 710 views light from retroreflector 711 re-reflected back frompanel 702 including defect 713 thereon.

As can be seen, light from the defect area is deflected away from thedirect reflectance angle θ by the sloped walls of the defect. Thisresults in a darker area `D` on the screen than would otherwise havebeen the case, and with light redistributed to create a brighter areaaround the dark area.

Because the distance L2 of the source to the defect is typically muchlarger than the defect size itself, the subtended illumination angle ofthe defect area is typically smaller--i.e. the illumination is nearlyparallel. Thus little "filling in" of the dark or light zones so createdoccurs, and the eye or other detector sees this effect. The zone `D` isnot completely black, however, as the eye is coincident with theillumination axis and the only light not returning to the eye is thatwhich is re-emitted by the screen over a nonzero angle γ and which hitsthe normal surface, for example, rather than passing right back throughthe same sloped surface of the defect.

The same sort of effect also occurs in the direction perpendicular tothe plane of the drawing.

Now let us consider the effect of placing the light source off the angleof view as with light source 720. In this case, much less light from thesloped edge of the defect farthest from the eye can reach the edge andit thus appears darker, accentuating the indication. This is desirablein many cases.

Let us think now of how the automatic sensing of the invention can beutilized in stamping, molding or body plants.

FIG. 13 of the referenced U.S. Pat. No. 4,394,683 illustrates panelscoming along the line. This is a typical arrangement for fixed sensorslooking at panels coming off a press. In other installations, however,the panel might be in stationary motion or moving and a robotic arm isused to position the sensor unit. The checking of panels of this typecan be done in two ways according to the present invention. The arm canactually sweep the sensor unit if it is capable of good uniform motionor conversely motorized tooling at the end of the arm can be used tomake the sweep with the robot actually in a fixed position (which can beeasier to program). Similarly, because of the properties of theretroreflective material, a two axis sweep can be utilized where theunit scans the surface with a 2 mirror sweep that raster scans. Forexample, motor 260 drives a planar mirror 261 (dotted lines) in FIG. 3to provide a sweep in the y direction as well.

FIGS. 9 to 12 illustrate several applications to plant use. Depicted inFIG. 9 is a robot 730 (in this case a gantry type Westinghouse 6000) anda scanned single sensor unit 732 comprising a retroreflective materialsender and receiver along the lines of FIG. 3. This sensor canoptionally further employ a scanning capability in the y axis using a 2axis mirror scan or conversely a motor drive on the end effector toolingof the robot. The robot can be programmed using programming consol 734to inspect numerous different types of panels 736.

The signal data from the sensor can be fed back to help maintain thestandoff distance from the part or additional sensors added for thispurpose if necessary.

FIG. 10 illustrates a multiple fixed sensor unit according to eitherFIG. 3 or FIG. 7 in which car doors 750 move on a conveyor 751underneath the sensor unit `nest` 752 on frame 753. In this case, anautomatic highlighter 755 is employed using a combined spray 760 andbrush 765 operation.

Also illustrated in this figure is an automatic reject of defectivepanels to a robotic repair station 800. A robot 801, taking signals fromthe defect readout 802, picks up a disc grinder 805 or other tool andgrinds down and feathers the defect 810. After doing so, the panel isfed back to the inspection station and reinspected to determine if it isnow okay.

FIGS. 11a and 11b illustrate the use of robot mounted units 902 and 904to scan a complete body-in-white 908 at a fixed position on-line. Arobot highlighter 906 is employed using a brush/spray end effector 910coordinated with the scan to always present `streaks` if any, parallelto the direction of scan. A similar version can operate on finishpainted cars where no highlight is required. Note that inspection ofpanel gap and mismatch can also be accomplished using a light sectiontriangulation sensor carried by the robot as well.

FIG. 12 illustrates an in-line version of the present invention for useon finished (painted) cars comprising fixed sensors 920 positioned toview the car 922 in-line. The large standoff and range of theretroreflective sensor types is a big advantage here allowing linemotion to be cleared in most cases.

It is noted that this invention is useable not only on car body,appliance, and other panels to see defects thereon, but also on thedies, the wood models, clay models, molds and other formed parts orartifacts that are used in the sheet metal plastic and body buildingprocess. The invention is used to determine defects in form of theseproducts and keep them from being propagated into the finalproduct--e.g. the painted car.

Clearly, if one can see the small flat spots and other minute localizederrors of form in the dies, one knows therefore where to take off thematerial, and how much to take off in the quantitive sense to make thedie right. The same holds true even before the die process where woodmodels are used so that the models themselves can be checked to makesure that they don't have any errors which are then traced into machinesthat make the dies and resulting in great waste.

Clearly, to make the invention work, one has to have the surfacessufficiently reflective. This means coating the wood, clay or metal withsomething, either oil, wax, reflective paper or some other material thatcan make it sufficiently reflective. Generally, it is desired also thatthe coating material be easily removeable.

It is particularly interesting to see the local form errors of dies andthen look at the panels that are produced by them to correlate thedefects, etc.

In the FIGS. 6 and 7 apparatus, it is further noted that to suppress theeffect of ripple on the surface whether it be from orange peel,highlight oil, elephant hide on plastic, grain on plastic, or whatever,one can do several things:

1. Purposely blur the image as through defocussing. This is notnecessarily effective in all cases as some of the depth of focus is verylarge in this system.

2. Utilize an oscillating glass to purposely mechanically blur the imageby moving it. This effectively smears over the highlight on the screenmaking an average signal. However, it also can clearly move the radiantimages as well as can the previous blurring.

3. Use a diffusing screen through which the images are viewed and whichdoes not allow one to focus clearly on the highlight droplets.

4. Utilize (as in the FIG. 3 apparatus) computer filtering andprocessing to process the signals. For example, all lower frequencysignals can be removed through AC coupling and all high frequencysignals can be removed except those exhibiting certain characteristics,for example, showing the typical look of either a large deviation or aone sided or bi-directional slope of a dent.

In utilizing the invention, one can also make a rapid scan of thesurface in hardware to identify that there is a suspected presence of adefect and then analyze the same signal which has been digitized througha software program at a relatively more leisurely pace to make a betterevaluation. This can be going on while continued further sections of thepanel are being scanned since one does not expect to find too manydefects during the total scan.

Conversely, one can simply scan the panel and come back to those areaswith suspected defects and simply dwell on them. This in effect thendoes not require a memory since one can just sit over the defect onceit's found and analyze it. Since computer memory nowadays is cheap,however, it seems just as logical to read it in and keep going whileanalyzing it as the other data is being streamed in.

It is important to think of the possible ways of looking at this data.As one comes up next to a flawed area near a character line or what haveyou on the panel that one does not wish to see, one has to have some wayof stopping the scan of the unit so that this is not picked up as aflaw. This can be done by simply storing the computer coordinates of thezones on the panel which are not to be looked to and blocking those outin the memory after one reads the scan in.

The other thing that can be done is to simply use the edge of the scanto see such flaws and come in with a precise triangle wave fed signalthat allows one to back right up against the surface. Alternatively, onecan rotate the sensor head so that the scan is parallel to the characterline or what have you and scan across a flaw in that direction coming upright next to it. It is noted that with good highlighting or paintfinish, one does not have to worry too much about the scan direction andsuch rotation is quite feasible. It's only in the case where thehighlighting is poor and streaky that one really needs to scan parallelto the streaks.

To help the cause of highlight oil spreading out, one should, whereverpossible, have a time delay built in between the application ofhighlight oil and the inspection, preferably at least 10 seconds ormore.

It is noted that the retardation plates and polarizers are not asnecessary at the lower angles as they are at the higher angles utilizedfor best performance. In other words, at low angles direct reflectionback from the panel surface, be it paint or whatever, is less.

The processing described in FIG. 5a for seeing the rate of change ofslope has been successfully used in finding low spots as shallow as0.0002 inches (0.005 millimeters) in depth. Such low spots are, however,typically in the range of 0.0002 to 0.0025 inches in depth and generallythe size of between one inch (2.5 cm) and 4 inches (10 cm) in overallwidth.

FIG. 13 illustrates a computer printout according to the invention.

In operating the invention, it has been found that spreading the beam inthe scan direction using a cylinder lens, such as the optionallyprovided lens 240 shown as dotted lines in FIG. 3, spreads the beam inthe scan direction and helps to improve the performance on low spotswhile providing a further averaging effect on the highlight conditions.However, at the same time, use of such a cylinder lens tends to masksmaller defects such as small dirt pimples and the like. In this case,it can be desirable to have a system which makes a scan in one passusing a cylinder lens (or another method of spreading the beam) and onthe return pass does not use it, thereby giving two sensitivities, oroptical intergrations, in the direction of scan. Such a programmabledevice can be a solenoid to simply pull the cylinder lens in and out,or, at higher speed, an acousto-optical modulator to spread the beam onone pass and do nothing to it on the next.

It is also possible to provide such signal averaging manipulations inhardware circuits or computer software 290 as shown in FIG. 3. Hardwaresignal averaging can be used like that of U.S. Pat. No. 4,305,661 usingtapped analog delay lines which allow the instantaneous signal to becompared to the average of sections of signal spaced ahead and/or behindin time.

A programmable correlator can also be used to correlate the signals ofthe different defects to actual signals. For example, low spots, dings,and dents all have the positive and negative going slope signals but atdifferent widths. Therefore, while the second derivative circuit workson those where the slope is high, those of less slope can be obtainedfrom correlation, either using a hardware correlator or preferably onetuneable at different frequencies to allow the right match to the signalin question to be obtained. In addition, a computer software correlationcan be made if time permits.

Correlation is not the only way to see such signals but it does allowthe known signatures of the defect to be matched.

Relative to the visual and TV versions of FIGS. 6 and 7, it has beenfound that in some cases with the observer looking directly through withthe lights, either surrounding the eye or placed very near the eye, thatthis does not give as good a view as with the light slightly displaced,for example, in the vertical direction looking at the panel hood in FIG.6a.

For example, with a vertical displacement H as shown in FIG. 6b, let ussay with the light for example 2 inches above or below the eye, thelight power coming back from the retroreflector is considerablydecreased at let us say L2=10 feet away because one is off the retroangle somewhat. However, there is a definite shadowing effect that takesplace under these conditions which tends to accentuate the defects,often providing a clearer view (as the direct view can wash out in somecases).

For some purposes, it could be desirable to switchably view the flawwith the light along the axis and at an off-axis position. In this case,two sets of lights can be used: one central, and one off axis. The twosets of lights are then simply switched. This switching can be automaticor manual. Conversely, two TV cameras can be used with a single lightwith the two cameras spaced, for example, and switched.

FIG. 14 illustrates a circuit capable of defect discrimination in theFIG. 3 embodiment, which is used to generate the readout of FIG. 13.

As shown in FIG. 14, the returned laser light is imaged on the UDT PinSpot 2D photodetector (photodetector 215 in FIG. 3) typically forming aspot. The detector's output currents are converted to voltage in thefirst AD644 halves. The voltages are then amplified by the second set of644's as well as being combined. Two outputs applied to the 4291 HDivider are the "sum" of the light striking the detector, and the"difference" between the halves of the detector.

The divider's output (Difference/sum) is the power compensated"position" of the light spot on the detector.

The spot position signal and the beam steering mirror's (mirror 202 inFIG. 3) position signal are both sent to the Rack board for furtherprocessing.

The mirror's position is differentiated to give the COS of mirrorposition. This signal is then applied to a zero crossing detector toobtain a mirror "direction" signal. The original SIN signal is sent toan Analog to Digital converter (ADC) so that the computer can read theposition of the mirror. A "position balance" potentiometer is used tocorrect for small delays through the differentiator and the "enablewidth" control allows digitization of only a part of the mirror's swing.

The image spot position signal is sent to a zero crossing detector aswell as a differentiator. The zero crossing signal is sent to a pair ofmonostables, used to generate a pulse on every zero-crossing no materwhich direction.

The differentiated spot position signal is sent to an absolute valueamplifier. This stage's output is applied to an ADC to allow thecomputer to read the apparent "severity" of the defect on the surfacebeing inspected. This "severity" signal is then compared to a thresholdwhich is computer generated from a DAC. The comparator's output is thenused to gage the zero crossing pulses. Only when the spot positionsignal is crossing through zero and the differentiated position issufficiently large, does the "Defect found" flip flop get set.

The computer then reads the "mirror position" and the "severity" fromthe circuit, and stores these values as well as the "polarity" and scanline number into a data array for further processing.

The "polarity" signal is generated by exclusive OR-ing the "mirrordirection" and a signal generated by comparing the "differentiated spotposition" with zero volts.

Because of the quirk in the inner workings of the ADC's it is necessaryto apply two pulses in quick succession to their clock inputs in orderto cause a conversion, hence the extra monostables and gates.

The computer generates a list of flaws giving the x and y locations, theseverity, the type (in or out dents) the flaw length and a rating basedon length "severity". The "severity" is then plotted against xycoordinates.

What is claimed is:
 1. A method of inspecting a surface comprising thesteps of:illuminating an extensive area of the surface by directinglight onto the surface area in such a manner that light is reflectedtherefrom; providing a retroreflective member comprising a large numberof small retroreflective elements in a position such that lightreflected from the extensive illuminated surface area impinges thereon,is then returned to the illuminated surface area, and is re-reflectedtherefrom; imaging light re-reflected from the extensive illuminatedsurface area; and detecting dark or bright areas in the imaged light,the dark or bright areas being indicative of a characteristic of theextensive illuminated surface area.
 2. A method according to claim 1wherein said illuminating is effected with light from a divergent lightsource.
 3. A method according to claim 2 wherein said divergent lightsource comprises a point light source.
 4. A method according to claim 1wherein said imaging and detecting are effected by visual observation.5. A method according to claim 1 further comprising recording saidimaged light.
 6. A method according to claim 5 wherein said detectingcomprises detecting said dark or bright areas in said recorded imagedlight.
 7. A method according to claim 2 wherein said illuminating iseffected from a location displaced from the axis of light detection. 8.Apparatus for inspecting a surface comprising;illuminating means forilluminating an extensive area of a surface by directing light onto thesurface area in such a manner that light is reflected therefrom; aretroreflective member comprising a large number of smallretroreflective elements positioned relative to the surface such thatlight reflected from the extensive illuminated surface area impingesthereon, is then returned to the illuminated surface area, and isre-reflected therefrom; imaging means for imaging light re-reflectedfrom the extensive illuminated surface area; and detecting means fordetecting dark or bright areas in the imaged light, the dark or brightareas being indicative of a characteristic of the extensive illuminatedsurface area.
 9. Apparatus according to claim 8 wherein saidilluminating means comprises a divergent light source.
 10. Apparatusaccording to claim 9 wherein said divergent light source comprises apoint light source.
 11. Apparatus according to claim 8 furthercomprising means for recording said imaged light.
 12. Apparatusaccording to claim 11 wherein said detecting means comprises means fordetecting said dark or bright areas in said recorded imaged light. 13.Apparatus according to claim 8 wherein said illuminating means ispositioned in a location displaced from the axis of light detection. 14.A method of inspecting a surface comprising:illuminating an extensivearea of a surface of an object with light from a divergent light source;providing a retroreflective member comprising a large number of smallretroreflective elements in a position such that light reflected fromsaid illuminated surface is returned to the illuminated surface andre-reflected therefrom; and imaging said re-reflected light.
 15. Amethod according to claim 14 wherein said imaging is effected by visualobservation.
 16. A method according to claim 15 wherein said visualobservation is effected from a vantage point which is substantially onthe axis of illumination of said surface.
 17. A method according toclaim 15 wherein said visual observation is effected from a vantagepoint which is displaced from the axis of illumination of said surface.18. A method according to claim 14 further comprising recording theimaged light.
 19. A method according to claim 14 wherein said divergentlight source comprises a point light source.
 20. Apparatus forinspecting a surface comprising:light source means for illuminating anextensive area of a surface of an object with divergent light; aretroreflective member comprising a large number of smallretroreflective elements, said member being positionable such that lightreflected from a surface illuminated with divergent light from saidlight source means is returned to the illuminated surface andre-reflected therefrom; and means for imaging said re-reflected light.21. Apparatus according to claim 20 wherein said imaging means ispositioned at a vantage point substantially on the axis of illuminationof said surface.
 22. Apparatus according to claim 20 wherein saidimaging means is positioned at a vantage point displaced from the axisof illumination of said surface.
 23. Apparatus according to claim 20further comprising means for recording the imaged light.
 24. Apparatusaccording to claim 20 wherein said divergent light source comprises apoint light source.
 25. A method of visually observing a geometricdistortion in a surface comprising:illumiating an extensive area of asurface of an object; providing a retroreflective member comprising alarge number of small retroreflective elements in a position such thatlight reflected from said illuminated surface is returned to theilluminated surface and re-reflected therefrom, said re-reflected lightcomprising bright or dark areas indicative of a geometric distortion insaid surface; and visually observing light re-reflected from theilluminated surface to observe the surface and a geometric distortiontherein.
 26. A method according to claim 25 wherein the step of visuallyobserving said surface is effected from a vantage point which issubstantially on the axis of illumination of said surface.
 27. A methodaccording to claim 25 wherein the step of visually observing saidsurface is effected from a vantage point which is displaced from theaxis of illumination of said surface.
 28. A method according to claim 25wherein said illuminating if effected with light from a divergent lightsource.
 29. A method according to claim 28 wherein said divergent lightsource comprises a point light source.
 30. Apparatus for renderingvisible a geometric distortion in a surface comprising:means forilluminating an extensive area of a surface of an object; and aretroflective member comprising a large number of small retroreflectiveelements; said retroreflective member being positioned in a locationsuch that light reflected from said illuminated surface is returned tothe illuminated surface and re-reflected therefrom, said re-reflectedlight comprising visible bright or dark areas indicative, under visualobservation, of a geometric distortion in said surface.
 31. Apparatusaccording to claim 30 further comprising means for recording an image ofsaid re-reflected light comprising said bright or dark areas. 32.Apparatus according to claim 31 wherein said image recording means ispositioned at a vantage point which is substantially on the axis ofillumination of said surface.
 33. Apparatus according to claim 31wherein said image recording means is positioned at a vantage pointwhich is displaced from the axis of illumination of said surface. 34.Apparatus according to claim 30 wherein said illuminating meanscomprises a divergent light source.
 35. Apparatus according to claim 34wherein said divergent light source comprises a point light source. 36.A method of detecting a geometric distortion in a surfacecomprising:illuminating a surface of an object with light such that anarea of such surface is illuminated, said area being substantiallylarger than a geometric distortion to be detected; providing aretroreflective member comprising a large number of smallretroreflective elements in a position such that light reflected fromsaid illuminated surface is returned to the illuminated surface andre-reflected therefrom; and detecting light re-reflected from theilluminated surface, said re-reflected light comprising detectable darkor bright areas indicative of a geometric distortion present in theilluminated surface.
 37. A method according to claim 36 wherein saidobject comprises an automotive body panel and wherein substantially theentire area of said body panel is illuminated.
 38. A method according toclaim 36 wherein said illuminating is effected with a divergent lightsource.
 39. A method according to claim 38 wherein said divergent lightsource comprises a point light source.
 40. A method according to claim36 wherein said detecting comprises visually observing said re-reflectedlight.
 41. A method according to claim 36 wherein said detecting iseffected from a vantage point displaced from the axis of illumination ofsaid surface area.
 42. A method according to claim 36 wherein saiddetecting comprises recording an image of said re-reflected light. 43.Apparatus for detecting a geometric distortion in a surfacecomprising:means for illuminating a surface of an object with light suchthat an area of said surface is illuminated, said area beingsubstantially larger than the area of a geometric distortion to bedetected; a retroreflective member comprising a large number of smallretroreflective elements; said retroreflective member being positionedsuch that light reflected from said illuminated surface is returned tothe illuminated surface and re-reflected therefrom; and means fordetecting light re-reflected from the illuminated surface, saidre-reflected light comprising detectable dark or bright areas indicativeof a geometric distortion present in the illuminated surface. 44.Apparatus according to claim 43 wherein said illuminating meanscomprises a divergent light source.
 45. Apparatus according to claim 44wherein said divergent light source comprises a point light source. 46.Apparatus according to claim 43 wherein said detecting means ispositioned at a location displaced from the axis of illumination of saidsurface.
 47. Apparatus according to claim 43 wherein said detectingmeans comprises means for recording an image of said re-reflected light.48. A method of inspecting a surface comprising:illuminating a surfaceof an object with light; providing a retroreflective member comprising alarge number of small retroreflective elements in a position such thatlight reflected from said illuminated surface is returned to theilluminated surface and re-reflected therefrom; and detecting lightre-reflected from the illuminated surface, said illumination beingeffected from a location displaced from the axis of light detection. 49.A method according to claim 48 wherein said detecting comprises visuallyobserving said re-reflected light.
 50. A method according to claim 48wherein said illuminating is effected with a divergent light source. 51.A method according to claim 50 wherein said divergent light sourcecomprises a point light source.
 52. A method according to claim 48wherein said detecting comprises imaging said re-reflected light anddetecting dark or bright areas in the imaged light.
 53. A methodaccording to claim 52 further comprising recording the imaged light. 54.Apparatus for inspecting a surface comprising:means for illuminating asurface of an object with light; a retroreflective member comprising alarge number of small retroreflective elements; that saidretroreflective member being positioned such light reflected from saidilluminated surface is returned to the illuminated surface andre-reflected therefrom; and means for detecting light re-reflected fromthe illuminated surface; a location said illuminating means beingpositioned at displaced from the axis of light detection.
 55. Apparatusaccording to claim 54 wherein said illuminating means comprises adivergent light source.
 56. Apparatus according to claim 55 wherein saiddivergent light source comprises a point light source.
 57. Apparatusaccording to claim 54 wherein said detecting means comprises means forimaging said re-reflected light and for detecting dark or bright areasin the imaged light.
 58. Apparatus according to claim 57 furthercomprising means for recording the imaged light.